Temperature and strain monitoring during thermoforming of thermoplastic composite laminates using optical frequency domain reflectometry

In this study, optical frequency domain reflectometry (OFDR) was used to monitor the thermoforming processes of carbon fiber reinforced thermoplastics (CFRTPs) to address the limitations of conventional sensors including large size and low spatial resolution. A bare single-mode fiber with a polyimide coating and a fiber encapsulated by a long metal capillary were cascaded and embedded into composite laminates to withstand the high pressure and temperature during thermoforming, and then connected to the OFDR for monitoring. A fiber encapsulated by a 2 cm short metal capillary was also embedded to demonstrate that a 1 mm resolution of the OFDR is beneficial for reflecting the local change in the composite. After processing by wavelet denoising, signal extraction, and decoupling, the frequency shift along the optical fiber sensor was successfully converted to strain and temperature. In two repeated thermoforming experiments that involved cooling from 340 °C, the average temperature difference measured by the OFDR and reference thermocouple was only 4.64 °C. The strain measured by the OFDR and reference fiber Bragg grating (FBG) decreases in the cooling stage, and has a clear knee point of 250 °C when correlated with the temperature and strain. This knee point is consistent with the liquid–liquid transition temperature of the polyetherimide and indicates the beginning of consolidation when the composite changes its properties significantly. The average strain difference measured by OFDR and the reference FBG was 69 μϵ when the total strain is approximately 1820 μϵ if only considering the consolidation process from 250 °C. The results of 1 mm spatial resolution and high accuracy demonstrate that OFDR is a promising high-resolution sensing solution for the in-situ temperature and strain monitoring of the thermoforming of CFRTPs.